This invention relates to the field of signal generators, and in particular signal generators for producing decoy signals to a homing torpedo. In even more particularity, the subject invention relates to a signal generating method using basic Digital Signal Processing (DSP) techniques to effectively enable Counter-Measures (CM) so that the torpedo can be avoided.
The days have long since passed when a submarine launched three or four torpedoes, each separated by half a degree of bearing, in order to ensure at least one hit on the target. Modern torpedoes incorporate powerful sonar seekers with advanced signal processing and computer technology to search for (and destroy) their targets. These torpedoes are faster and more maneuverable than any submarine or surface ship. While early acoustic torpedoes relied upon a target""s radiated noise, today""s torpedo uses its own active sonar to detect the target or the targets wake. The most advanced of these weapons uses target motion information from the Doppler shift of its sonar ping to eliminate false targets and increase the signal to noise ratio for better detection.
Torpedo sonar systems operate at relatively high frequencies (compared to a search sonar) in order to obtain the resolution necessary for target classification. One trade off is that higher frequency means higher absorption of the sonar energy in seawater, which limits the detection range of the weapon. Frequently the target knows that a torpedo has been launched and has time to initiate evasive action. Torpedo countermeasures are then deployed to prevent the torpedo from detecting and homing on the evading ship. While there are many different kinds of torpedo countermeasures employed by the world""s navies, they all try to accomplish two things: attract the torpedo""s attention (present a false target) and reduce the torpedo""s ability to detect the true target.
Most of these countermeasure devices are simple noisemakers, programmed to produce noise at a high sound pressure level. Just as it is difficult to carry on a normal conversation when there is loud music playing nearby, these countermeasure devices increase the ambient noise level, which in turn decreases the torpedo""s signal to noise ratio and lowers the probability that the torpedo will detect the target.
The major drawback to noisemakers is that their signal typically remains the same during its entire active lifetime. A particular countermeasure may be very effective against one particular type of torpedo but may not be effective against another. However, there is usually no way for a noisemaker to change its output after it has been deployed from the target ship, particularly since acoustic communications are blocked by the noise itself. There are over two dozen kinds of torpedoes available on the open market, manufactured by countries such as the U.S., U.K., Germany, Italy, France, Russia, China, and North Korea. All of these weapons have very different sonar characteristics, which makes it difficult to produce a noisemaker that will be effective against them all.
Digital signal processor (DSP) technology as used in the present invention solves many of the problems associated with prior countermeasure devices. The overall goal of the present invention is to provide a torpedo countermeasure design that is superior in both performance and cost effectiveness when compared to previous countermeasures. A primary consideration is use of inexpensive commercial DSP technology in a countermeasure design that is able to recognize and properly respond to torpedo sonar signals.
The entire electrical/electronic equipment package is contained within a single waterproof housing of sufficient strength to operate at tactical water depths. An external mechanical flotation system is attached and can be preset prior to launch to maintain the encased equipment package at the appropriate water depth. Further, electrical and electronic connections penetrate the waterproof hardware casing, thus permitting battery charging and complete updates of software and firmware, so that the acoustic and tactical capabilities of the device can be improved over time with minimal expense and minimal hardware modifications.
An object of the present invention is to overcome the disadvantages of the prior art by providing a torpedo countermeasure that produces a signal that attracts the technically sophisticated homing torpedoes.
An object of the invention is to provide a countermeasure that uses a single transducer to detect a torpedo sonar ping, analyze its characteristics (frequency, pulse length, etc.), and, using the transducer, transmit a return pulse that will be interpreted by the torpedo as the refection off a valid target.
Accordingly, a system according to the invention for producing a decoy signal to enable a target to avoid a homing torpedo that uses a sonar ping signal for homing in on the target comprises a transducer arranged to operate in a receive mode in which it receives sonar signals and produces corresponding electrical signals and in a transmit mode in which it emits a sonar return pulse. The invention further comprises a digital signal processor connected to the transducer. The digital signal processor is arranged to analyze the electrical signals corresponding to sonar signals received by the transducer and to determine whether they were emitted by the homing torpedo. The digital signal processor is further arranged to switch the transducer to the transmit mode in response to receipt of a sonar ping signal from the homing torpedo and to cause the transducer to transmit a return pulse that acts as a decoy signal to the homing torpedo.
The system according to the invention preferably further comprises an analog to digital converter connected to the transducer to receive analog signals therefrom and to output corresponding digital signals and a filter connected between the analog to digital converter and the digital signal processor. The filter is arranged to determine whether signals input thereto from the analog to digital converter are within a defined frequency range.
The system according to the invention preferably further comprises a digital to analog converter arranged to receive digital signals output from the digital signal processor and produce corresponding analog signals and an amplifier connected between the digital to analog converter and the transducer.
The invention preferably further comprises a front-end filter connected to the transducer and arranged to pass signals having frequencies less than a predetermined frequency, an analog to digital converter connected to receive signals passed by the front-end filter and to output corresponding digital signals and a filter connected between the analog to digital converter and the digital signal processor, the filter being arranged to determine whether signals input thereto from the analog to digital converter are within a defined frequency range.
A method according to the invention for producing a decoy signal to enable a target to avoid a homing torpedo that uses a sonar ping signal for homing in on the target comprises the steps of (a) receiving sonar signals with a transducer that produces electrical signals corresponding to the sonar signals, (b) determining whether the electrical signals have frequencies that are within a defined frequency range (c) processing the electrical signals to determine whether they correspond to sonar ping signals, (d) calculating an estimate of the torpedo""s motion, (e) calculating a transmit time for transmitting a decoy signal, (f) calculating a Doppler frequency for the decoy signal such that the decoy signal will appear to the homing torpedo to be a return pulse from the target and (g) switching the transducer to a transmit mode to transmit the decoy signal having the calculated Doppler frequency at the calculated transmit time.